In contemporary crystal engineering halogen bonding is booming due to its directionality, specificity and high strength allowing preparation of highly complex structures with a high degree of accuracy and precision. Surprisingly, yet being the most difficult to achieve, hollow molecular assemblies solely based on halogen bonding have only very recently been achieved. Our group and Francois Diederich (ETH, Zürich) both published, in 2015 and nearly simultaneously, the first molecular capsules solely based on halogen bonds. Even though halogen bonding is more directional than hydrogen bonding it is even more sensitive to the environment and has thus mostly been demonstrated in the solid-state. Studies on halogen bond donor cavitands and collaboration with Mate Erdelyi (University of Gothenburg, Sweden, at present Uppsala University) on halonium ion based halogen-bonded complexes based on [N···Ag+···N] to [N···I+···N] cation exchange reaction (Fig. 1) inspired us to exploit halonium ions as very robust halogen bond donors. The first supramolecular, halonium ion mediated, dimeric capsule is based on a resorcinarene-based, conformationally fixed cavitand that is functionalized with four adjacent meta-pyridyl units acting as the XB acceptors. Assembly of the two cavitands first with silver(I) cations followed by a subsequent reaction of the dimeric silver capsule with molecular iodine leads to [N···I+···N] halogen-bonded dimeric capsule through [N···Ag+···N] to [N···I+···N] cation exchange reaction.8 Most recently we have managed to create an extremely robust large, even hexameric, halogen-bonded molecular capsules with the help of [N•••I+•••N] halogen bonds.
Fig 1. Syntheses of the dimeric Ag+ bridged and I+ bridged dimeric capsules.